As well known in the art, a vehicle such as an automobile makes the driving such as accelerating, decelerating and turning runs by transmitting its torque using the frictional force between its wheels and the road surface. In other words, the vehicle cannot transmit the torque over the frictional force between the wheels and the road surface. For stable runs, therefore, the driving torque or the braking torque has to be kept within such a range as to establish no slip of the wheels.
The driving torque is raised either by increasing the output of a power source such as an internal combustion engine or by augmenting the gear ratio. On the other hand, the braking torque rises according to the depression of a brake pedal or the gear ratio in the decelerating state. In either case, the torque is generally controlled by the manual operation of a driver. However, the friction coefficient of the road surface on which the vehicle is driving changes every moment, and the driving torque or the braking torque based on the manual operation may exceed the torque which is determined by the friction coefficient of the road surface. In order to keep a stable running even in this case, the driving torque or the braking torque is controlled not by the manual operation.
One example of the control for suppressing the driving torque is the so-called “traction control”, and one example of the control for suppressing the braking control is one in the device called “anti-lock brake system (ABS)”. One example of the anti-lock brake system is described in Japanese Patent Laid-Open No. 58-26661. When the engine output is raised for starting or accelerating the vehicle on a road surface of a low friction coefficient (or the so-called “low μ road”), for example, the idle rotation or slip of a wheel is detected in the former control by a speed sensor. The idling or slipping wheel is braked by feeding the oil pressure to its brake actuator, and the output torque is lowered by delaying the ignition timing of the engine. In the latter control for avoiding the locking of a wheel, on the other hand, the locked state of the wheel is detected on the basis of the vehicle speed and the wheel speed of the case in which a braking operation (or braking) is done, and the braking force on the locked wheel is lowered by releasing the brake actuator for the wheel, so that the braking torque is made lower than the value determined on the basis of the road friction coefficient to keep the gripping force of the wheel.
Either of these is a control corresponding to the so-called “wheel control means” for suppressing the rotation of the wheel or for lowering the braking force to stop the rotation of the wheel. When the wheel control means of this kinds acts, therefore, the load torque in the drive line from the engine to the drive wheel rises or drops on the contrary. In the contents of the control, however, when the driving torque or the braking torque is relatively higher than the road friction coefficient, the rotation of the wheel is kept while keeping the grip of the road surface by suppressing the torque acting on the wheels. Even in the case of an abrupt braking on a low μ load, therefore, the rotation of the wheel can be kept to continue the rotation of the engine connected to the drive wheel thereby to avoid or suppress from inviting an engine stall.
When the aforementioned device (or system) for controlling the rotation of the wheel fails or has a malfunction in its control, however, the rotation of the wheel may abruptly drop or may be stopped. As the rotation of the drive wheel thus changes, the speed of the engine is abruptly lowered to highly possibly invite the engine stall as the vehicle on which the aforementioned wheel control means such as the ABS is not mounted. On the other hand, the abrupt drop in the speed of the drive wheel is caused when a high braking force acts on the drive wheel so that a transmission to which the drive wheel is connected is subjected to such a high torque as to stop the rotation of the transmission. As a result, the transmission may be damaged by an excessive torque when the braking is done with the aforementioned wheel control means being in the state of failure or malfunction. In the transmission such as a continuously variable transmission (CVT) in which the torque is transmitted by a frictional force or a shearing force of an oil film, for example, the rotary member such as a belt or pulleys may be highly possibly damaged.
Thus, the device (or system) such as the ABS for controlling the rotation of the wheel can keep the rotation of the wheel, when it normally functions, but exerts serious influences to the entirety of the drive line when it comes into a failure or malfunction. However, the control has not been clarified in that of the prior art that the failure or malfunction, if any, can be properly coped with.
This invention has been conceived in view of the background situations thus far described, and has an object to provide a control device which can keep the rotation of a power source even when a vehicle is braked with means for controlling the rotation of a wheel being troubled by a failure.